1. Field of the Invention
The present invention relates to aryl-substituted aliphatic hydrocarbons, and more particularly to methods for preparation of aryl-substituted aliphatic acids from unsaturated aliphatic hydrocarbons and aromatic hydrocarbons.
2. Description of the Prior Art
Conventional methods for preparation of aryl-substituted aliphatic acids suffer from several drawbacks. For example, standard methods not only have involved production of complex reaction mixtures and required expensive and cumbersome separation techniques, but also result in low yields. Such aryl-substituted aliphatic acids are particularly useful as corrosion inhibitors or as substrates for corrosion inhibitors. Saturated fatty acids have been considered for such uses; but they are ordinarily solids which are insoluble in the oil products, such as motor oil, for which the inhibitors are desired. Thus, branched acids, such as isostearic acid, sometimes have been used. However, due to the drawbacks of the conventional methods for preparation of branched acids, such acids are expensive.
Typically, a Friedel-Crafts reaction has been the vehicle for preparation of aryl-substituted saturated acids. According to this reaction, an aromatic hydrocarbon, such as benzene or toluene, is reacted in the presence of an acidic reagent with an unsaturated aliphatic acid, for example, oleic acid. The acidic reagent employed in this process is a strong Lewis acid, commonly aluminum chloride, boron trifluoride or hydrofluoric acid. For example, in Nakano, Y. and Foglia, T. A., "Methanesulfonic Acid Catalyzed Addition of Aromatic Compounds to Oleic Acid," JAOCS, Vol. 61, No. 3 (March 1984), the use of aluminum chloride to catalyze a reaction between benzene and oleic acid is noted.
Such methods involve several disadvantages. For example, since the Lewis acids employed in the Friedel-Crafts reaction are extremely strong acids and are extremely reactive, and because hydrogen halide gas is generated, special handling and extra safety precautions are required. In addition, the reaction is exothermic and must be moderated by cooling. Further, the Lewis acid is consumed in the reaction, making recycling of the acid impossible and adding significantly to the cost of the reaction. Moreover, such methods produce inferior yields when the aliphatic acid is internally unsaturated.
In addition, the reaction also is associated with several undesirable side reactions, resulting in limited yields and separation problems. Morever, for example, since one such side reaction is polyalkylation, a substantial excess of the aromatic hydrocarbon is commonly employed in an effort to limit polyalkylation of the aromatic ring. However, such efforts to limit polyalkylation have proven unsatisfactory since use of a substantial excess of the aromatic hydrocarbon tends to increase costs of the process and, in any event, polyalkylation is not entirely eliminated by such measures.
According to other side reactions, where an acid such as oleic acid is employed as an aliphatic hydrocarbon substrate in the reaction, the intermediate carbonium ion reacts with the carboxyl group of the acid to form lactones. If an unsaturated acid is employed as the aliphatic hydrocarbon substrate, the intermediate carbonium ion may also react with two acid molecules to yield an ester of the acid. As a result of the involvement of such side reactions, the yield of the desired product is diminished and the reaction product comprises a mixture of compounds, requiring extensive purification techniques to isolate the desired mono-substituted alkyl aromatic.
Moreover, many procedures required by the conventional reaction prove very troublesome in practice. The reaction products are darkly colored, making separation and isolation difficult; and steam distillation is often necessary to isolate the desired products. In addition, the products obtained are complex mixtures; and their complex structure adds to the difficulty in purification. Not only that, but also the excess highly reactive Lewis acid which is employed in the reaction as a catalyst must be carefully quenched.
Further, the physical properties of the reaction product are often not suitable for the desired applications. For example, the typical reaction yields a high melting solid, when commercial application, for example, as a corrosion inhibitor or substrate therefor, requires a low melting solid or a liquid. Thus, an improved method for preparation of suitable branched fatty acids is needed.
The Nakano and Foglia article noted above describes a method for adding aromatic compounds to the double bond of oleic acid by a methanesulfonic acid catalyzed reaction. However, the reaction described therein requires copious amounts of methanesulfonic acid (6:1 molar ratio based on oleic acid content) and produces yields significantly under 80%, resulting in a difficult separation process involving a two-step extraction/water washing technique and a relatively expensive process. Moreover, the significant interplay of undesirable side reactions in the Nakano and Foglia method results in a product containing large amounts of impurities, especially lactones and esters of oleic acid. Accordingly, the industry is still searching for satisfactory methods for preparation of aryl-substituted aliphatic hydrocarbons.